Phonon tweaks

[utf]

Optimise phonon workflow.

Main changes:

• Displacement k-point density dropped down.
• Use auto_ispin.
• Use prev_vasp_dir when possible to enable auto_ispin.

@JaGeo I'm currently running some tests to make sure the results are still reasonable.

Fixes #271.

[JaGeo]

Great, thanks. We could also add a line in the documentation that k-point settings need to be carefully tested before using the workflow in high-throughput production as there is not yet a large-scale test.

[codecov]

Codecov Report

Merging #276 (ce99a22) into main (02e44c0) will increase coverage by 0.00%.
The diff coverage is 95.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #276   +/-   ##
=======================================
  Coverage   64.96%   64.96%           
=======================================
  Files          74       74           
  Lines        7196     7199    +3     
  Branches      946      947    +1     
=======================================
+ Hits         4675     4677    +2     
- Misses       2221     2222    +1     
  Partials      300      300           

Impacted Files	Coverage Δ
src/atomate2/settings.py	95.45% <ø> (-0.11%)	⬇️
src/atomate2/vasp/jobs/phonons.py	80.00% <50.00%> (-0.90%)	⬇️
src/atomate2/vasp/flows/phonons.py	89.24% <100.00%> (+0.35%)	⬆️

[JaGeo]

Is it fine to merge from your side, @utf ? I will work on the phonon workflow next week and it would be great if this gets merged before to not deal woth merge conflicts.